1. Field of the Invention
The present invention relates to a process of making a wear resistant shoe of the type frequently employed to engage a swash-plate or moving cam surface in a pump or similar device, and the product resulting from that process.
2. Description of the Related Art
It is a fairly common practice to harden machinery parts that are subject to substantial wear. For example, pumps (and analogous motors) sometimes employ a plurality of pistons that are sequentially actuated by a off-axis rotating cam surface called a swash plate. Each piston has a shoe which is fixed to the piston by a ball and socket joint and has wear resistant face which engages the swash plate. U.S. Pat. No. 5,728,475 illustrates a technique for hardening the wear surfaces of a shoe suitable for use in such a pump, motor or similar environment which is compatible with aircraft fuel.
In this patented arrangement, the shoe is machined to a nearly final shape and then coated to provide a surface hardening. Thereafter, a skirt portion of the surface hardened shoe is annealed so that the skirt may be crimp attached to a rounded portion of the piston to form the ball and socket joint. This technique has met with good success, however, for more demanding applications, an improved wear couple may be required. In some applications, the boride surface hardening coating tended to crack, apparently because the hard thin boride coating was insufficiently supported by the soft basis metal. In addition, some operating conditions caused wear-through of the coating. The need to crimp the shoe to the piston head precludes the use of starting stock in the cold-reduced condition. The decreased ductility in this condition would result in greater chance of cracking during crimping.
It is desirable to prevent cracking of the surface hardening and to increase the wear resistant properties of such a shoe.
The present invention provides a shoe with a work hardened foundation, a surface hardened face and a work hardened skirt forming the socket of the ball and socket joint. A cold heading process of cold working to form the cam engaging surfaces and a crimping process of cold working to join the piston and shoe are performed as separate process steps. If the entire cylinder blank from which the shoe is formed were cold worked, the crimping process would be difficult or impossible to perform without cracking. This process differs from U.S. Pat. No. 5,728,457 in that the hardening of the cam engaging surfaces (a balance land and back flange) is achieved by cold heading as opposed to a thermal diffusion boriding.
In general, and in one form of the invention, a wear resistant shoe is made by cold-heading one end portion of a generally cylindrical blank to radially increase and axially diminish the dimensions of the one end portion, and to work harden the one end portion while leaving an opposite end portion dimensionally unchanged. Subsequent cold-working and hardening of the opposite end portion is achieved while joining the shoe and piston during crimping of the skirt about a received piston rounded end. A hollow skirt is machined in the dimensionally unchanged portion prior to the subsequent cold-working.
Also in general, a wear resistant shoe is formed from hardened rod stock and assembled to a piston by first annealing one end portion of the machined rod stock and then forming a hollow region in the annealed rod stock end portion. Subsequent crimping of the periphery of the hollow region about a rounded end of the piston rehardens the annealed end.
An advantage of the present invention is that a hardened region provides support for a surface hardened face to reducing fracturing of the surface hardened face. No Laves phase precipitation occurs; thus no post hardening thermal treatment is required. Cold heading of the shoe through hardens, as opposed to case hardening the upset material, thus simplifying grinding and lapping of the balance land and auxiliary balance lands. Wear resistance of the auxiliary balance land increases. A subsequent surface hardening, for example, a 2 micron thick titanium nitride coating, remains intact after testing under conditions that historically wore through the 25 micron thick boride coating and into the basis metal. Masking operations can be performed more simply by mechanical arrangements as opposed to the chemical means employed in former process. The harder substrate provides a superior base on which to support the wear resistant coating.